The present invention generally relates to communications systems and, more particularly, to a receiver.
In the ATSC (Advanced Television Systems Committee) standard for digital terrestrial television (DTV) in the United States (e.g., see, United States Advanced Television Systems Committee, “ATSC Digital Television Standard”, Document A/53, Sep. 16, 1995), the modulation system consists of a suppressed carrier vestigial sideband (VSB) modulation with an added small in-phase pilot at the suppressed carrier frequency, 11.3 dB below the average signal power, at the lower VSB signal edge. An illustrative spectrum for an ATSC VSB signal is shown in FIG. 1.
A typical ATSC-VSB receiver includes a carrier tracking loop (CTL) that processes a received ATSC VSB signal to both remove any frequency offsets between the local oscillator (LO) of the transmitter and LO of the receiver and to demodulate the received ATSC VSB signal down to baseband from an intermediate frequency (IF) or near baseband frequency (e.g., see, United States Advanced Television Systems Committee, “Guide to the Use of the ATSC Digital Television Standard”, Document A/54, Oct. 4, 1995; and U.S. Pat. No. 6,233,295 issued May 15, 2001 to Wang, entitled “Segment Sync Recovery Network for an HDTV Receiver”). In this regard, the CTL performs carrier phase estimation.
One category of methods for performing carrier phase estimation in an ATSC-receiver is referred to as “blind” or non-data aided. Blind methods rely on either the ATSC pilot, or on a pseudo-pilot created by subjecting the lower VSB signal edge to some form of processing such as squaring. Unfortunately, the main problem with using a blind approach is that under certain transmission channel conditions, such as ghosting, the lower band edge of the ASTC VSB signal is either completely destroyed or distorted in such a way as to make it impossible to extract the desired carrier offset information.
Another category of methods for performing carrier phase estimation in an ATSC-receiver is referred to as “non-blind” or data aided. Data aided methods rely on (partial) knowledge of the information (symbols, bits) being transmitted. However, a data aided approach, especially in the ATSC context, generally can't be used by itself and, instead, is used in an auxiliary capacity to, e.g., maintain carrier lock once it has been acquired through the use of a blind method.